Linear accelerator (“linac” hereinafter) X-ray generating systems have been in use in the medical environment for a number of years. More recently, such systems have begun to be used in the industrial environment, particular for cargo scanning. To distinguish the materials inside a cargo container, X-ray pulses of different energies have been used. Energies in the range of 2 MeV to 10 or more MeV have been proposed in the literature, and commercial devices offering energies at approximately 4 MeV and 6 MeV are commercially available. In many instances, both medical and industrial linac X-ray sources are excited by RF sources operating at or near the S-band, roughly 2.998 GHz. In each case, the linac accelerates a stream of electrons in conjunction with RF excitation, and the linac must be designed such that its resonance can be matched by the frequency of the RF source. Once the electrons have been sufficiently accelerated, if X-rays are desired, they strike a target, such as tungsten, resulting in the emission of high energy X-rays that can be used for medical treatment, materials processing, scanning of cargo, and other applications. These mega-electron-volt (MeV) X-ray applications have provided significant benefits in many fields.
The most prevalent medical and industrial linacs are resonant structures, and require an RF excitation source, typically either a pulsed magnetron or a pulsed klystron. To couple the power from an RF excitation source into a linac, for the purpose of accelerating electrons, the frequency of the RF source output must be adequately matched to the resonance frequency of the linac structure. Standing wave linear accelerators are somewhat more sensitive to the accuracy of the excitation frequency than are traveling wave accelerators, yet both are sensitive. In addition, a given linac's optimum excitation frequency is typically sensitive to temperature of the linear accelerator, as is well known. The output frequency of the RF excitation source can also change with environmental conditions such as temperature, and AFC (automatic frequency control) circuits are often used to maintain a good match between the RF source and the linac.
The maximum electron energy, and thus the resulting X-ray energy that can be obtained from a given linac structure is dependent upon the peak power coupled into it from the RF source, and also dependent upon the beam current within the accelerator at the time the RF power is applied, using known relationships. In addition, the Q of the linac can affect the performance of the system. For most linac-based systems, a high Q has been deemed desirable, typically between 5000 and 10,000.
In industrial applications such as cargo scanning, X-ray pulsing permits images of the contents of a container to be created without opening the container. The high energy, MeV level X-rays from linac systems allow adequate penetration through large containers and their typical contents. The image of the contents of a container is typically a composite of a large number of scans, or image “slices” at different energies, in some cases 1000 to 10,000 or more such slices. As an example, a commercial cargo scanning system may pulse a linac at 100 to 400 X-ray pulses per second, as a truck passes through a scanner, and the images from each of those pulses are then composited to create the completed image. In some prior art cargo scanning operations, imaging systems that use linacs can scan between 24 and 150 trucks per hour depending upon the mode.
Different X-ray energies, such as 4 MeV and 6 MeV, are useful in cargo scanning because it permits the materials in the container to be differentiated. By comparing the images taken at 4 MeV with those taken at 6 MeV, steel can be distinguished from uranium, as just one example. Likewise, organic material, aluminum, lead, and so on, can be distinguished.
However, it is impractical to take a first scan of a container resting on the bed of a truck at a first X-ray energy, and then take a second, later scan of that same container at a second X-ray energy. Instead, it is more practical to interleave pulses of the different frequencies, so that a scan of both energy levels is taken in a single pass, for example by means of interleaved pulses of two energies in an ABABABAB pattern. This interleaving, however, can present challenges to a linac system since the performance of a linac with its resonant structure is highly dependent on a good frequency match. Some well known techniques for modifying the input parameters to a typical 6 MeV linac system, in order to obtain a 4 MeV output, involve either reducing RF power to reduce the maximum possible acceleration for electrons in the linac, or increasing the beam current to provide beam loading that ultimately reduces the maximum acceleration that each electron experiences, or a combination of these two effects. These techniques have been sufficient when the time between energy changes is long, such as seconds or many seconds, but have been less effective when the time between energy changes is a fraction of a second. It is also possible to intentionally detune the resonance frequency of the RF source with respect to the linac—this has the effect of lowering the amount of RF power coupled into the linac, reducing the total possible acceleration for electrons. However, detuning can cause instabilities in performance, and is difficult to do in a fraction of a second if a mechanically-tuned magnetron is the RF source. A challenge with applying prior art techniques for changing linac energy on a rapid time scale in an ABABABAB fashion can be the undesired corresponding detuning of the RF source. For example, in addition to intentional tuning, reducing or increasing RF power from a magnetron typically results in a change in frequency of the RF output. If the frequency change is large enough, the linac's resonance is no longer matched to the frequency of the RF excitation pulse, and the system fails to operate. A pulse-to-pulse variation of RF amplitude or variation in RF frequency in a detuned condition will cause a greater change in linac output performance than operation at peak or tuned condition.
While the terms “4 MeV” and “6 MeV” are commonly used in the art, those terms typically refer to the peak energies of the X-ray pulses, and the average energy can be less. These terms will be used herein with that same understanding.
Because of the sensitivity of linac-based systems to changes in frequency, the prior art has generally not been able to provide a fully optimized train of stable, interleaved pulses of different energies such as an ABABABABAB (etc.) pattern, especially where the energy must be changed rapidly, from one pulse to the next, at a 2.5-millisecond basis or shorter such as required for a pulse rate of 400 or more pulses per second, and especially when that rapid change is done with a magnetron driven system.
While magnetrons do not offer some of the advantages of klystrons, magnetrons such as the MG5193, MG6090, and MG7095 from supplier e2V have typically been used as RF sources for medical and cargo scanning applications. Similar magnetrons are available from National Japan Radio Corporation (NJRC). Unlike klystrons, magnetrons are not amplifiers, and the output frequency of a magnetron is adjusted by a mechanical tuner. This limits the ability to rapidly switch a magnetron system between higher and lower energy levels while still maintaining a frequency match with the linac, because the magnetron frequency will shift upon a power change and a mechanical tuner simply cannot be moved rapidly enough to support a 2.5 millisecond or shorter period between pulses. The duty cycle of such devices is typically low, for example approximately 0.1% for the MG5193, MG6090, and MG7095, such that pulse durations of a few microseconds, for example 2.5 microseconds, can be generated at up to 400 pps. For an MG5193, 4.5 turns of the mechanical tuner allow for about nine megahertz of tuning range. One drawback of a magnetron is that its mode and stability can become unfavorable if the magnetron is operated at a peak power too much different from its optimum or maximum peak power. The MG5193 can operate between about 1.5 MW peak and about 2.6 MW peak. The MG6090 and MG7095 can provide higher peak power outputs, such as 3 MW of peak power.
It is possible to change the power output of a magnetron rapidly through the use of a conventional high voltage capacitor-charging power supply together with a pulse forming network/modulator. With such a technique, interleaved high voltage (“HV”) pulses can be supplied to the magnetron in an ABABABAB sequence. The result is that the RF power output of the magnetron can be varied rapidly as well, and also in an alternating fashion.
However, as noted before, changing the power output of the magnetron also causes a change to the output frequency of the RF pulse, such that the output frequency of the magnetron can be a mismatch for the linac. For example, the MG5193 and similar magnetrons can have a frequency shift of about 10 kHz per ampere, and they are typically driven at around 100 amps or more. Changing this current by many amperes may cause a significant detuning with respect to the resonance of a given linac. While AFC circuits can compensate for long term changes in frequency, such circuits are not intended to compensate for instantaneous pulse-to-pulse changes such as occur with a magnetron is driven with an alternating sequence that varies by several amps. As a result, such approaches can result in instability of the magnetron-linac system, and there has been a need for a system and technique for maintains stability while permitting the use of interleaved pulses of different RF powers to generate an interleaved pattern of X-ray pulses of different energies.
In addition to the challenges to prior art systems that result from variations in RF matching between the output frequenct of a magnetron and the resonant frequency of the linac, the prior art has also had challenges in maintaining a consistent dose from pulse to pulse when interleaved pulses of different energies are generated. While dose control is known in medical systems, the need for ABABABAB sequences of different energies is not typically found in medical X-ray systems. Dose control in prior systems typically involves either changing the RF peak power coupled into the linac, or through beam loading, which involves changing the peak electron beam current into the linac, or both. For example, it is well known that increasing the peak beam current into an accelerator will reduce the energy of the electrons leaving the linac. The value of the beam current can be controlled by controlling the electron gun, but, if the maximum energy of the electron beam is change, then the dose rate will also change absent taking steps to prevent that change. For example, using beam loading to change the X-ray energy from 6.5 MeV to 5 MeV can change the dose in some systems by a factor of two. However, for the use of X-rays to differentiate among materials, as required for cargo scanning applications, such changes in dose per pulse are undesirable. Instead, it is preferable that both energy and dose be controlled on a pulse-by-pulse basis even where the pulses are at different energies, such as energies that are different by more than 1 MeV.
While dose control in medical applications can be achieved through changes in repetition rate, this is not desirable for cargo scanning applications which depend upon having consistent repetition rates. As a result, there has been a need for a cargo scanning system which offers dose control on a pulse-by-pulse basis while offering stable sequences of interleaved pulses of different energies.
A further problem in cargo scanning applications is the intermittent nature of the operation of such scanning systems. The X-ray emission in a cargo scanning system is typically turned off after each scan is completed. For example, a first truck, carrying a container, can be scanned, and then the scanner is turned off. A next truck carrying a container arrives a few seconds later, or a minute, or some other indeterminate time period. As noted previously, the operating frequency of the linac changes with temperature. It is well known that the linac heats up during scanning, which requires that the output of the magnetron be adjusted to maintain a good match with the linac. While AFC circuits, which typically rely on feedback of the forward and reflected power of the linac, can maintain that good match when the linac system is being pulsed, there is no such feedback when the scanner is off and the linac starts to cool. As a result, typical prior art systems using conventional AFC may leave the tuner of the magnetron in the same position it was in when the scanner was turned off. Depending upon the frequency drift of the linac versus the RF source during the “off” period and the mismatch between RF source frequency and linac resonance, the system may simply not operate when scanning is restarted, or, more commonly, the energy output and/or dose output of the linac system will be less than intended. While this affects only the first few pulses, or 10 pulses or more after scanning is re-initiated, before the AFC circuit can achieve a good match, this lack of consistency in output energy and/or dose can affect the quality of the resulting images, and is therefore undesirable.
One prior art technique involves turning on the RF source to the linac in advance of enabling any beam current from the electron gun of the linac, to perform a partial warm-up of the linac beamline by the RF source prior to the electron beam triggering. In another approach, a typical AFC is used during the RF-only partial warm-up period, with an offset added to the tuning in advance of any expected electron beam triggering.
These warm-up approaches have the significant disadvantage of potentially generating some amount of X-rays even when the electron gun is not being pulsed. Such X-rays can be generated by virtue of the high electric fields in the RF-pulsed linac pulling electrons from the electron gun and cavity walls. Another disadvantage is the consumption of average power that is not used for the generation of X-rays. These approaches are inefficient and therefore undesirable.
As a result, there has been a need for a linac-based X-ray scanning system that provides consistent pulse energies even for the initial pulses of a sequence despite intermittent operation and “off” periods of indeterminate duration.